Electrostatic chucks, also known as platens, are used to secure and support a workpiece for processing. Electrostatic charge may accumulate on the workpiece and also on the platen surface supporting the workpiece. Charge accumulated on the workpiece can be transferred to the platen surface, and charge accumulated on the platen surface can be transferred to the workpiece.
Charge accumulation can cause workpiece “sticking” problems. In one instance, the charge accumulation can be so great that typical unclamping forces can not release the workpiece. For example, a typical unclamping force may be provided by raising lift pins to contact the backside of the workpiece. In another instance, the lift pins may be able to lift a portion of the workpiece but a remaining portion remains in contact with the workpiece. When the workpiece is a disk shaped semiconductor wafer, the wafer may become “tilted” appearing stuck to an edge of the platen. When an associated robot arm attempts to retrieve the wafer, it may not properly engage the wafer and even push the wafer off the platen leading to possible wafer damage and interruptions in processing. Another workpiece “sticking” problem may be referred to as “dancing around the platen.” In this instance, the workpiece may be clamped to the platen during processing or in a load position. With some circular or disk shaped workpieces such as a semiconductor wafer, the wafer may enter into a general oscillatory precession around the outer circumference of the platen creating great risk for dropping of the wafer. In other cases, “wafer walk” may occur, which may be caused by partial sticking of the wafer edge to the platen during lifting, resulting in a wobble of the wafer on the lift pins and possible misalignment on the pins leading to a wafer handling problem.
Even if charge accumulation does not lead to workpiece “sticking” problems, it may lead to damage of devices being formed on the workpiece. In a plasma doping ion implanter where the workpiece is positioned in the same chamber as plasma, excessive charge accumulation can also lead to doping non-uniformities, micro-loading, and arcing. Hence, the throughput of the plasma doping ion implanter may be intentionally limited in some instances to avoid excessive charge accumulation.
One conventional solution to controlling charge accumulation uses three spring loaded grounding pins that contact a backside of the workpiece to provide a path to ground when the workpiece is in a clamped position. One drawback of this solution is that the spring loaded grounding pins are limited to three pins. As such, the effectiveness of this grounding arrangement to dissipate excessive charge build up is limited. Another drawback of this solution is that the contact points of the spring loaded grounding pins have sharp edges that can cause damage to the backside of the workpiece. Damage to the backside of the workpiece can also generate unwanted particles (contamination) which may be critical to limit in some processing applications.
Accordingly, there is an ongoing need to improve the performance of electrostatic chucks.